1. Field of the Invention
The present invention relates generally to implantable cardiac stimulators, and more particularly to a capacitor charging circuit in an implantable defibrillator.
2. Background Information
Implantable defibrillators are implanted in patients who can be identified as being likely to suffer cardiac arrhythmias, such as ventricular fibrillation, that can cause sudden death. The defibrillator detects the occurrence of ventricular fibrillation and automatically delivers defibrillating therapy. Implantable defibrillators in their most general form include appropriate electrical leads for collecting electrical signals generated by the heart, and for delivering electric shocks to the heart to provide defibrillation therapy. Also included are batteries, energy storage capacitors, and control circuitry connected to the leads, batteries and capacitors for sensing the electrical activity of the heart and for charging the capacitors and triggering the delivery of shocks through the leads. Implantable defibrillators can also include circuitry for providing cardioverting therapy for treating tachycardia, and for providing pacing therapy for treating bradycardia.
Defibrillation therapy generally involves rapid delivery of a relatively large amount of electrical energy to the heart at high voltage. Presently available batteries suitable for use in implantable defibrillators are not capable of delivering energy at such levels directly. Consequently, it is customary to provide a high-voltage energy storage capacitor that is charged from the battery via appropriate charging circuitry. To avoid wasting battery energy, the high-voltage energy storage capacitor is not maintained in a state of charge, but rather is charged during an interval after fibrillation has been identified by the control circuitry, and immediately prior to delivering the shock.
The charging circuitry in an implantable defibrillator sometimes involves switching circuitry for cyclicly interrupting DC current flow from the battery through the primary winding of a voltage step-up transformer in order to induce a transient current in the secondary winding of the transformer during the fly-back period. The induced fly-back current in the secondary winding is rectified and applied to the terminals of a high-voltage energy storage capacitor, thereby causing the energy storage capacitor to become fully charged over a number of switching cycles.
It is desirable that energy be transferred from the battery to the storage capacitor as efficiently as possible in order to reduce the time required to charge the high voltage storage capacitor. This avoids excessive delay in delivering therapy after fibrillation has been detected by the implantable defibrillator. Capacitor charging speed is affected by the fact that the voltage delivered by the battery to the charging circuitry drops over the life of the battery. In a conventional, fixed switching frequency charging circuit, as the battery voltage drops, the average current drawn from the battery during charging also drops over the life of the battery, resulting in an increase in the time required to charge the energy storage capacitor.
It is also desirable that current flow from the battery to the capacitor be limited during the initial charging period to avoid saturation of the step-up transformer and excessive current flow through the switching device. At the time that charging begins, the storage capacitor is initially in a state of discharge. Due to the volt-second product imbalance between the primary and secondary windings of the transformer, high current will be drawn from the battery until voltage starts to build up in the energy-storage capacitor.